There is a wide variety of refrigerants used in air conditioning equipment depending on the application.
In general the most common refrigerants used in the industry belong to the following three categories -

  • CFC - These are the Chloro Fluoro Carbon refrigerants, such as R11, R12, R113, R114, etc. These
    refrigerants were identified as the most harmful to Ozone layer by the Montreal Protocol, and
    were phased out in 2000. However they are still being used in the older machines, with
    precautions to minimize release in accordance with EPA regulations. The most common application
    of these refrigerant is in the large centrifugal chillers. R12 was also used commonly in the older
    cars for air conditioning.
  • HCFC - These are the Hydro Chloro Fluoro Carbon refrigerants, such as R22, R123, etc. These
    refrigerants were identified as slightly harmful to the Ozone layer by Montreal Protocol, and will
    be completely phased out by 2030. See the EPA link below for the different stages of the
    phaseout. The R22 refrigerant is commonly used in reciprocating type of compressors, while R123
    is used in centrifugal chillers as a temporary replacement for R11.
  • HFC - These are the Hydro Fluoro Carbon refrigerants, such as R134a. These are the new
    refrigerants that do not harm the Ozone layer, and are being used in the newer machines to
    replace the CFC and HCFC. R134a is now commonly used as a replacement of R12 and R500, and in
    all new cars air conditioning systems. R407c is used as a replacement for R22. One of the other
    common HFC used in new equipment now is R410a.

There is extensive research going on to identify new refrigerants that can be used to replace the CFC
and HCFC refrigerants. The various refrigerants have different characteristics, which make them
suitable for a particular application.
Following link provides more useful information on EPA regulations
the various refrigerants - EPA

Refrigerant Testing & Analysis

The refrigerant tends to get contaminated with time in the refrigeration unit due to moisture, heat,
wear of mechanical parts, oil degradation, etc. It needs to be tested periodically to ensure that it is
performing optimally and not damaging the compressor and other components. If necessary it may have
to be cleaned
depending on the test analysis, before further use. Information on the test analysis
required for optimum performance of various refrigerants is provided by the link
given below.